Added new html/LocalStorage test

dart/html/LocalStorage.dart

+#import('../../../../../dart/client/testing/unittest/unittest.dart');
+#import('dart:html');
+
+main() {
+  forLayoutTests();
+  test('GetItem', () {
+    final value = window.localStorage.getItem('does not exist');
+    Expect.isNull(value);
+  });
+  test('SetItem', () {
+    final key = 'foo';
+    final value = 'bar';
+    window.localStorage.setItem(key, value);
+    final stored = window.localStorage.getItem(key);
+    Expect.equals(value, stored);
+  });
+}